A strong, level concrete base is essential for a sectional garage. Errors in its size, level or drainage can prevent the panels and doors from aligning correctly, while poor ground preparation may lead to cracking or movement later.
This guide explains the main stages involved in laying a base for a concrete sectional garage. It provides a useful overview, but the exact dimensions, depth, concrete specification and reinforcement must be confirmed for the garage and site before work begins.
Working with wet concrete carries serious risks. Cement can cause chemical burns, particularly when trapped inside gloves or footwear. Wear suitable waterproof gloves, eye protection, long clothing and wellington boots, and avoid kneeling or standing in fresh concrete.
Step 1: Confirm the Garage Specification
Do not prepare the base using the nominal garage dimensions alone. Sectional garages may require additional space around the walls for brackets, fixings or a foundation unit.
For many Dencroft garages, the required base size includes extra space beyond the external dimensions of the building. Use the measurements supplied with your quotation, survey or order rather than assuming that a standard allowance will suit every model.
Confirm the following details before excavation begins:
- The exact length and width of the base
- The required depth and concrete specification
- Whether reinforcement is needed
- The finished base level
- The position and opening direction of the garage door
- Any personnel doors, ramps or drainage channels
- When the base must be completed before installation
The base should also be considered as part of the wider garage installation process, including access for delivery vehicles and the removal of any existing structure.
Step 2: Assess the Site and Drainage
Choose a position that provides enough room for the garage, doors, installation team and delivery equipment. Check for trees, overhanging branches, retaining walls and neighbouring buildings that could restrict construction or future access.
Before digging, establish whether underground cables, drains, pipes or other services cross the area.
Remove turf, topsoil, organic material and other unsuitable ground. Concrete should not be poured directly over vegetation because it can decay and leave unsupported areas beneath the slab.
Soft ground, made-up ground, significant slopes or sites close to retaining walls may need specialist design. Adding mesh without addressing unsuitable soil will not correct the underlying problem.
Plan Where Rainwater Will Go
The finished arrangement should direct surface water away from the garage rather than allowing it to collect around the panels or base. The surrounding ground, driveway gradient, gutters and door threshold all affect the drainage plan.
A channel drain or other drainage system may be required where a driveway slopes towards the garage. A gravel trench should not automatically be treated as a suitable soakaway. Ground permeability, nearby foundations, boundaries and the proposed discharge point must be considered first.
Step 3: Prepare and Compact the Sub-Base
Most garage bases require a properly compacted granular sub-base beneath the concrete. Its depth and material should be selected to suit the ground conditions and expected loads.
Lay the sub-base evenly and compact it in appropriate layers using suitable equipment. It should provide consistent support across the entire slab, without soft spots or loose areas.
If reinforcement is specified, it must be the correct type and positioned at the required depth using suitable supports. Steel mesh left directly on the ground will not provide the intended reinforcement. Fibres added to concrete are not automatically a substitute for structural mesh, so any change must be approved by the person specifying the base.
Consider a Damp-Proof Membrane
A damp-proof membrane can help reduce moisture passing from the ground into the garage floor. Whether one is needed, and where it should be positioned, depends on the base design and intended use of the building.
The membrane should cover the required area without punctures and be detailed correctly around edges and joints. Its purpose is to control ground moisture, not to prevent uneven curing of the concrete.
Step 4: Install the Shuttering
Shuttering forms the outside edges of the slab and must be strong enough to resist the pressure of wet concrete. Securely brace it with suitable stakes and fixings before the pour begins.
Check that the shuttering is:
- Set to the exact specified dimensions
- Square, with matching diagonal measurements
- Level at the required finished height
- Firmly supported without movement
- Accessible for the planned delivery method
A sectional garage normally requires a level base so that its panels, roof and doors align correctly. Do not introduce a fall across the slab unless the garage manufacturer has specifically approved it. Drainage should instead be addressed through the surrounding levels and properly designed channels.
If a ramp is needed at the entrance, agree its dimensions and position before pouring. An incorrectly positioned ramp can interfere with the garage door or direct water towards the building.
Step 5: Order Suitable Ready-Mixed Concrete
Provide the concrete supplier with accurate dimensions and explain that the concrete is for a garage base. The required strength, consistency and any admixtures should be specified by the supplier, groundworker or project designer.
Calculate the required volume carefully and allow for reasonable site variation. For larger bases, ready-mixed concrete is usually more practical than mixing small batches because it allows the slab to be placed consistently.
Plan the delivery before the vehicle arrives. Consider:
- Whether the lorry can safely reach the site
- Whether concrete will be discharged directly, pumped or transported by wheelbarrow
- How many people are required for placing and finishing
- Where surplus concrete and washout will be contained
- Whether the forecast temperature, rain or wind could affect the pour
Do not add water to delivered concrete simply to make it easier to place. Unauthorised additional water can reduce its strength and durability. If greater workability is required, discuss the correct consistency or admixture with the supplier when ordering.
Step 6: Place and Finish the Concrete
Place the concrete as close as practical to its final position and spread it evenly using appropriate tools. Avoid dropping or repeatedly moving it over long distances, as this can cause the materials within the mix to separate.
The concrete should be properly compacted to remove trapped air, particularly around edges and reinforcement. A straight screed or tamp can then be drawn across the shuttering to establish the finished level.
Once excess surface water has disappeared and the concrete is ready, use a float to produce the required finish. Avoid overworking the surface or adding dry cement or water, as this can contribute to a weak, dusty or cracked surface.
Contain concrete spillages and collect them before they harden. Do not hose concrete, cement residue or wash water into surface drains, watercourses or onto surrounding soil.
Step 7: Protect and Cure the Base
Concrete needs to retain moisture while it gains strength. Begin the agreed curing method as soon as the finished surface is firm enough not to be damaged. This may involve suitable plastic sheeting or another curing system recommended by the concrete supplier.
Protect the new base from:
- Rapid drying caused by warm weather or strong wind
- Heavy rain marking or washing out the surface
- Frost and freezing temperatures
- Foot traffic, vehicles and stored materials
- Damage when removing the shuttering
Concrete continues gaining strength over time, and its commonly specified strength is normally assessed at 28 days. The time required before garage installation depends on the concrete mix, slab design, weather and expected load. Do not assume that seven days will always be sufficient, particularly during cold weather.
Confirm the installation date with the garage manufacturer and concrete supplier, and do not erect the building until the base has cured sufficiently.
Common Garage Base Mistakes
- Using the garage’s nominal dimensions instead of the supplied base plan
- Pouring over turf, loose ground or an inadequately compacted sub-base
- Failing to check underground services before excavation
- Allowing water to collect around the base or door opening
- Adding water to ready-mixed concrete without approval
- Positioning reinforcement directly on the ground
- Overworking the finished surface
- Failing to protect the concrete during curing
- Installing the garage before the slab has gained sufficient strength
Professional Concrete Garage Base Laying
A garage base must be accurate enough for the sectional panels and doors to fit correctly. If you are uncertain about ground conditions, drainage, reinforcement or finishing the concrete, using an experienced groundworker can prevent costly installation problems.
Dencroft offers a complete garage base laying service designed around the size and specification of the chosen building. To discuss a site survey, base preparation or a complete garage installation, contact Dencroft Garages for advice and a quotation.
